Method for hardening groove walls of the ring groove of a steel piston

ABSTRACT

A method for hardening groove walls of the ring groove of a steel piston by a laser beam consists of conducting process gas that contains oxygen onto a groove wall of the steel piston, and putting the steel piston into rotation. The groove wall is irradiated with the laser beam until an oxide layer has formed on the groove wall. Subsequently, the feed of the process gas is turned off, and the groove wall is heated and hardened, in tracks. The degree of heat conductivity of the groove wall is improved by the oxide layer on the groove wall, thereby bringing about faster heating and hardening of the groove wall. The reflectivity of the groove wall is also reduced in this way, so that only little light is reflected by the groove wall onto other regions of the groove, so that these regions are not unintentionally heated and hardened.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. 119 of German Application No. 10 2007 054 736.8 filed Nov. 16, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for hardening groove walls of the ring groove of a steel piston.

2. The Prior Art

German Patent No. DE 103 37 962 A1 describes a method for laser hardening of groove walls of the ring grooves of a steel piston, in which a laser beam having a wavelength of less than 5 μm is used in order to achieve a degree of absorption of the laser beam energy that is suitable for hardening. The laser beam furthermore has to be directed at the groove to be hardened at a sufficiently great incidence angle. It is disadvantageous, in this connection, that there is only a small selection of laser sources that generate laser radiation at a wavelength of less than 5 μm. Furthermore, the selection of a sufficiently great incidence angle is limited by the geometry of the ring groove, i.e. by the groove wall that lies opposite the groove wall to be hardened, depending on the position of the laser source relative to the steel piston.

SUMMARY OF THE INVENTION

It is therefore on object of the invention to avoid these disadvantages of the state of the art. This object is achieved by a method for hardening groove walls of the ring groove of a steel piston that put into rotation about its piston axis, by means of a laser beam is directed at a groove wall. The method includes the following steps:

-   -   conducting process gas that contains oxygen onto the groove wall         of the steel piston,     -   putting the steel piston into rotation,     -   irradiating the groove wall surrounded by the process gas with         the laser beam, until an oxide layer has formed on the groove         wall,     -   turning off the feed of the process gas, and     -   heating and hardening of the groove wall by the laser beam, in         tracks.

In one embodiment, a mixture of argon and oxygen is used as the process gas. Alternatively, a mixture of nitrogen and oxygen can be used as the process gas. Or, a mixture of nitrogen, argon, and oxygen can be used as the process gas. Preferably, at least approximately 1% oxygen is mixed into the process gas.

Because the groove wall surface of the steel piston that is to be hardened is colored with iron oxide, its heat conductivity is increased to such an extent, and furthermore its reflectivity is reduced so greatly that the laser light can be radiated onto the groove wall surface to be hardened at any desired angle of incidence, independent of its wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention.

In the drawings, wherein similar reference characters denote similar elements throughout the several views:

FIG. 1 shows a device for laser hardening of the wall of a groove for a piston ring; and

FIG. 2 shows an enlarged representation of the piston ring groove, the lower wall of which is being hardened by a laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the drawings, FIG. 1 shows a laser source 2 attached to a holder device 1, which source, as shown in an enlargement in FIG. 2, directs a laser beam 3 at the lower wall 10 of the upper ring groove 11 of a piston 5 made of steel, for an internal combustion engine. Ring groove 11 is provided for a compression ring not shown in the figures. Steel piston 5 is mounted on a turntable 6, by way of a clamping holder 12, to rotate about its piston axis 13. Turntable 6 is attached to a foundation 7 having an electrical drive device not shown in the figure, by which the turntable is put into rotation, under electrical control.

Furthermore, a gas line 8 is attached to holder device 1 and to a stand 9, by which line process gas is passed in the direction of upper ring groove 11, by way of a gas jet 4, which particularly surrounds the region of lower wall 10, which is to be laser-hardened, with the process gas.

A diode laser or an Nd:YAG laser can be used as the laser source 2, for example. Using this, in the present exemplary embodiment of the invention, lower wall 10 of ring groove 11 for the compression is hardened, because this wall is exposed to increased stress during engine operation, by the compression ring. However, any surface of piston 5 that is subject to great mechanical stress can be hardened using the method according to the invention.

In this connection, the small focus of laser beam 3 heats lower surface 10 of ring groove 11 of piston 5 that is put into rotation by means of turntable 6, in tracks. A cooling medium for bringing about the cooling of the heated track that is required for the hardening process is not necessary, since very rapid temperature equalization between the heated track and the adjacent piston material, steel, occurs, bringing with it automatic quenching, which brings about good hardening of lower groove wall 10.

During this hardening process, the process gas is passed onto lower groove wall 10 by way of gas jet 4; it can consist of a mixture of argon and oxygen, of nitrogen and oxygen, or of argon, nitrogen, and oxygen. Within the scope of comprehensive experiments, good results were achieved with a process gas mixture consisting of 1% oxygen and 99% nitrogen.

The groove wall surface hit by laser beam 3 colors as the result of oxide formation, whereby iron oxide (Fe₂O₃) is formed on the surface. As a result, an improvement of the heat conductivity of the surface irradiated by the laser occurs, for one thing, thereby accelerating the heating of this surface and thus improving the degree of effectiveness of the laser. For another thing, the coloring of the groove wall surface by iron oxide reduces the reflectivity of this surface, thereby reducing the intensity of the laser beam 3′ reflected by groove wall 10 and shown with a dotted line in FIG. 2 to such an extent that groove root 14 hit by it is no longer sufficiently heated so that it is hardened. It is undesirable to harden groove root 14, i.e. the edge between groove wall 10 and groove root 14, because hardening of these regions reduces the basic impact resistance of the piston material in these regions to such an extent that cracks can form here during engine operation.

The method according to the invention for hardening groove wall 10 has the additional advantage that the angle of incidence α formed by groove wall 10 and laser beam 3 can be selected as desired, and is selected in such a manner that the size of the hardness penetration zone is optimally adjusted for the best possible hardening result.

Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.

REFERENCE SYMBOL LIST

-   α angle of incidence -   1 holder device -   2 laser source -   3, 3′ laser beam -   4 gas jet -   5 piston, steel piston -   6 turntable -   7 foundation -   8 gas line -   9 stand -   10 lower wall, groove wall -   11 upper ring groove -   12 clamping holder -   13 piston axis -   14 groove root 

1. A method for hardening groove walls of the ring groove of a steel piston that is mounted so as to rotate about its piston axis, by means of a laser beam directed at a groove wall, the method comprising the following steps: conducting process gas that contains oxygen onto the groove wall of the steel piston; putting the steel piston into rotation; irradiating the groove wall surrounded by the process gas with the laser beam, until an oxide layer has formed on the groove wall; turning off a feed of the process gas; and heating and hardening the groove wall by means of the laser beam, in tracks.
 2. The method according to claim 1, wherein the process gas consists of a mixture of argon and oxygen.
 3. The method according to claim 1, wherein the process gas consists of a mixture of nitrogen and oxygen.
 4. The method according to claim 1, wherein the process gas consists of a mixture of nitrogen, argon, and oxygen.
 5. The method according to claim 1, wherein at least approximately 1% oxygen is mixed into the process gas. 